1. Field of the Invention
The present invention relates to wireless sensor networks, and, more particularly, to wireless sensor networks that carry delay-critical messages.
2. Description of the Related Art
Collection of rare but delay-critical messages from a group of sensor nodes is a key process in many wireless sensor network applications. This is particularly important for security related applications like intrusion detection and fire alarm systems. An event sensed by multiple sensor nodes in the network can trigger the simultaneous transmission of many messages. In such cases, it is important that the first message be received quickly and that all the messages be eventually collected with low levels of latency.
With the transition of many automated tasks from a wired to a wireless domain, wireless sensor networks (WSNs) are being increasingly subjected to new application domains. Applications of critical nature have been the forte of wired networks due to wired networks' high levels of reliability. Despite the traditional use of wired networks, the ever-increasing reliability of WSNs coupled with their cost-effectiveness has led to the gradual adoption of WSNs for such critical applications as well. The nature of such applications, however, requires new Media Access Control (MAC) protocols for WSNs that meet the requirements and inspire sufficient confidence in the usage of WSNs.
The requirements for applications of critical nature can be fundamentally different from the applications for which known MAC protocols are designed. For example, energy is a valuable resource in sensor devices and most existing MAC protocols are optimized to conserve energy, trading off latency, throughput and other similar performance metrics in the process. These same protocols are typically not suitable when the application demands better performance at the expense of some additional energy. If reduction of latency is to be the primary issue with reduction of energy consumption being only a secondary issue, protocols need to be re-designed from that application perspective.
In some applications, wireless sensors are required to convey urgent messages to a centralized base station with minimum delay from the time the urgent messages are generated. These messages are triggered by events detected by sensor nodes, and the messages are intended to inform the base station so that the base station may take possible action. Such messages are triggered very rarely, and it may be desirable to reduce latency when such messages are triggered, even if some additional energy is expended during those times. Intrusion detection and fire alarm applications, for example, may require such low levels of latency. Even though the messages are typically correlated, the collection of all messages as opposed to just a subset of the messages provides valuable information which can be used for detection of false positives or post event analysis. For example, the European Standard EN 54-25 for fire alarm systems specifies the duration within which the first alarm should be reported and all alarms should be received at the base station. The challenges in designing WSN MAC protocols for such applications include the handling of a number of simultaneous messages without knowledge of how many messages will be simultaneously sent, and planning for possible interferences. Additionally, it is important to ensure implementation feasibility taking into account the additional constraints imposed on WSNs such as time synchronization and limited computation and storage capabilities.
MAC protocols for wireless sensor networks can be mainly classified into contention free, contention-based and energy saving protocols. Contention free protocols are mainly the ones based on Time-Division Multiple Access (TDMA) where slots are assigned to each node by the base station and each node sends its message (if it has one) only during its assigned slot. Such TDMA-based protocols perform very poorly when the number of nodes contending is unknown or keeps varying. In some applications, the number of nodes is rarely known, which makes TDMA-based schemes undesirable. Other contention free approaches, e.g., Frequency-Division Multiple Access (FDMA), face similar limitations.
Contention based protocols can be bifurcated into Carrier Sense Multiple Access-based (CSMA-based) or non-CSMA based. The IEEE 802.11 and 802.15.4 protocols are examples of CSMA protocols with the latter designed specifically for applications catered to by wireless sensor networks. These protocols use a variable sized contention window whose size is adjusted at each node based on the success of the node in sending its message, with each node picking a slot in this window using a uniform probability distribution. These protocols do a good job in handling scenarios with small number of nodes but do not handle a large number of simultaneous messages well. The Sift protocol was designed to overcome these deficiencies for WSN applications that need to handle such a large number of event-driven spatially co-related messages. Sift is also CSMA-based but uses a fixed size contention window. Nodes pick slots from a geometric probability distribution such that only a few nodes contend for the first few slots, and thus Sift handles a large number of messages easily. Protocols based on Aloha, on the other hand, do not sense the channel before transmission and rely on each node randomly picking a slot to transmit on, with the probability of transmission depending on the number of messages contending. When this number of contending messages is not known, these protocols do not adapt well. In general, CSMA-based protocols outperform-Aloha based protocols when the propagation time between nodes is small enough to make carrier sense useful.
What is neither disclosed nor suggested by the prior art is a method of handling delay-critical messages in a wireless sensor network with a reduced level of latency.